Directional drilling for the exploration and development of oil and gas fields advantageously provides the capability of generating boreholes which deviate significantly relative to the vertical direction (that is, perpendicular to the Earth's surface) by various angles and extents but generally follow predetermined profiles. In certain circumstances, directional drilling is used to provide a borehole which avoids faults or other subterranean structures (e.g., salt dome structures). Directional drilling is also used to extend the yield of previously-drilled wells by milling through the side of the previously-drilled well and reentering the formation, and drilling a new borehole directed so as to follow the hydrocarbon-producing formation. Directional drilling can also be used to provide numerous boreholes beginning from a common region, each with a shallow vertical portion, an angled portion extending away from the common region, and a termination portion which can be vertical. This use of directional drilling is especially useful for offshore drilling, where the boreholes are drilled from the common region of a centrally positioned drilling platform.
Directional drilling is also used in the context of substantially horizontal directional drilling (“HDD”) in which a pathway is drilled for utility lines for water, electricity, gas, telephone, and cable conduits. Exemplary HDD systems are described by Alft et al. in U.S. Pat. Nos. 6,315,062 and 6,484,818. HDD is also used in oilfield and gasfield exploration and development drilling.
A rotary steerable drilling tool is a type of directional drilling tool which allows for directional drilling of boreholes while allowing or maintaining rotation of the drill string. This technique can provide improved directional control, improved hole cleaning, improved borehole quality and generally minimizes drilling problems as compared to earlier technologies. Such tools include steering mechanisms enabling controlled changes in borehole direction. One type of steering mechanism involves expandable ribs or pads located around the drilling tool which can be actuated to apply a force on the borehole walls so as to direct the drilling tool in a desired direction. However, in part because they rely on contact with the borehole surface, such steering mechanisms can have certain disadvantages.